1. Field of the Invention
The present invention relates to an optical head, and an information recording-and-regeneration apparatus which includes the optical head.
2. Description of the Related Art
Among optical-information recording mediums with high density and a large capacity on the market, there is an optical disk called a DVD or a BD. Such an optical disk has been increasingly popular as a recording medium for recording an image, music or computer data.
As the capacity of an optical disk has become larger, the wavelength of an optical-head light source has become shorter. The numerical aperture of an objective lens has also become wider. However, the wider such an NA becomes, the more conspicuously a spherical aberration varies according to a change in the thickness of a light transmission layer in an optical disk. For example, in the case where the wavelength used in a DVD is 650 nm and its objective lens has an NA of 0.6, if the thickness of a light transmission layer is changed by 10 μm, a spherical aberration of approximately 10 m λ is produced. In contrast, a wavelength of 400 nm and an NA of 0.85 will be used for a next-generation optical disk. In that case, with respect to a change by 10 μm in a light-transmission layer thickness, a spherical aberration of about 100 m λ is produced. This is around ten times as long as that of a DVD.
As a means for correcting a spherical aberration, a system is described in Japanese Patent Laid-Open No. 11-259906 specification. A collimating lens is mounted in a collimating-lens actuator, and the collimating lens which is placed between a light source and an objective lens is moved. In this system, the collimating lens is moved, so that a spherical aberration which is caused by an error in the thickness of a light transmission layer can be cancelled. This will be specifically described below with reference to FIG. 9.
FIG. 9 shows the configuration of an optical head 101 which is disclosed in the above described specification. This optical head 101 includes: a light source 110; a diffraction grating 111; a polarization beam splitter 112; a collimating lens 113; a collimating-lens actuator 114; a quarter-wave plate 115; an objective lens 116; an objective-lens biaxial actuator 117; a multi-lens 118; and a photo-detector 119.
A beam of light, which is emitted from the light source 110, is first incident upon the diffraction grating 111. Then, it is diffracted by this diffraction grating 111. This diffraction grating 111 is used to split the beam of light at least into three, so that tracking servo control can be executed by a so-called three-spot method.
Then, a zero-order beam and a ±first-order beam (hereinafter, referred to, together, as the “incident beam of light”) which are formed after the beam of light has been diffracted by the diffraction grating 111 transmit the polarization beam splitter 112. Sequentially, they are incident upon the collimating lens 113. Herein, the collimating lens 113 is formed, for example, by getting two spherical lenses 113a, 113b to adhere to each other.
The beam of light, which is incident upon the collimating lens 113, turns into a parallel beam through the collimating lens 113. This is realized in the case where the thickness t of a light transmission layer 104 of an optical disk 102 is equal to a predetermined value.
Herein, this collimating lens 113 is mounted on the collimating-lens actuator 114. Thus, it can be moved back and forth along the optical axis of the incident beam of light by the collimating-lens actuator 114. Unless the light-transmission thickness t of the optical disk 102 is equal to a predetermined value, the collimating lens 113 is moved by the collimating-lens actuator 114 so that the spherical aberration which is caused by the thickness error of the light transmission layer 104 can be corrected. In other words, if the thickness t of the light transmission layer 104 of the optical disk 102 is not a predetermined value, the incident beam of light is transformed into a divergent beam or a convergent beam by the collimating lens 113, so that the spherical aberration caused by the thickness error of the light transmission layer 104 can be corrected.
Then, the incident beam of light, which is emitted from the collimating lens 113, is incident through the quarter-wave plate 115 upon the objective lens 116. Herein, when passing through the quarter-wave plate 115, the incident beam of light comes into a circularly-polarized light state. This circularly-polarized luminous flux is incident upon the objective lens 116.
The objective lens 116 is used to concentrate the incident beam of light on the recording layer of the optical disk 102. Specifically, the incident beam of light in the circularly-polarized light state through the quarter-wave plate 115 is collected by the objective lens 116. Then, it passes through the light transmission layer 104 of the optical disk 102 and is incident upon the recording layer of the optical disk 102.
The incident beam of light which is collected by the objective lens 116 and is incident upon the recording layer of the optical disk 102 is reflected by the recording layer. Thereby, it becomes a return beam. This return beam traces the former optical path and passes through the objective lens 116. Thereafter, it is incident on the quarter-wave plate 115. Then, the return beam transmits the quarter-wave plate 115, so that it becomes a linearly-polarized beam which is turned by an angle of 90 degrees to the polarization direction before the return. After this, the return beam is transformed into a convergent beam by the collimating lens 113. Thereafter, it is incident on the polarization beam splitter 112 and is reflected by this polarization beam splitter 112. The return beam, which is reflected by the polarization beam splitter 112, passes through the multi-lens 118 and is incident on the photo-detector 119. Then, it is detected by this photo-detector 119.
Using the above described optical head 101, recording and regeneration are conducted by concentrating a beam of light on the recording layer of the Optical disk 102. At that time, an aberration can be produced by an error in the thickness of the light transmission layer 104 in the optical disk 102. Such an aberration is mainly caused by a defocus and a spherical aberration.
A defocus is corrected by focus servo control. Specifically, focus servo control is executed based on the quantity of light which is detected in the photo-detector 119. Then, the objective lens 116 is moved forward and backward in the optical-axis directions by the objective-lens biaxial actuator 117. Thereby, a defocus is corrected so that the focus is adjusted onto the recording layer.
On the other hand, in terms of a spherical aberration, the incident beam of light which is incident upon the objective lens 116 is transformed into a divergent beam or a convergent beam. Thereby, a spherical aberration is generated which has an inverse polarity to a spherical aberration that is produced according to the thickness t of the light transmission layer 104. As a result, a correction can be made. Specifically, using the collimating-lens actuator 114, the collimating lens 113 is moved back and forth in the optical-axis directions. Thereby, the incident beam of light upon the objective lens 116 is transformed into a divergent beam or a convergent beam. Then, an inverse-polarity spherical aberration is generated by the objective lens 116. This makes it possible to cancel a spherical aberration which is caused by an error in the thickness of the light transmission layer 104. In other words, in this optical head 101, the collimating-lens actuator 114 works as a moving means for moving the collimating lens 113, so that a spherical aberration can be cancelled according to the thickness t of the light transmission layer 104 in the optical disk 102. Therefore, in this optical head 101, when a beam of light which is emitted from the light source 110 transmits the objective lens 116 to form a focal point, a spherical aberration remains cancelled. Hence, as the whole optical system, a spherical aberration is desirably corrected.
In order to realize a larger capacity in the future, it can be considered that the number of information recording layers should be increased. In other words, if more information recording layers are used to record information on each information recording layer, their capacity becomes larger. However, each information recording layer is located at a different distance from an objective lens. Hence, a spherical aberration to be generated is supposed to differ for each information recording layer. Specifically, let's assume that a residual spherical aberration of a beam of light, which reaches a certain information recording layer, is set to be at the minimum. At this time, the thickness of a light transmission layer is called an optimum material thickness. In this case, for an information recording layer different from this information recording layer, a spherical aberration is generated according to the length at which the thickness of a light transmission layer varies from the optimum material thickness. Herein, if a variation in the thickness of a light transmission layer is Δd, the refractive index of the light transmission layer is n and the numerical aperture of an objective lens is NA, then a third-order spherical aberration W is expressed by the following numerical formula (2) (Refer to pp. 60—in the second edition of optical disk technology by Radio Technology Co.).
                    W        =                                                            n                3                            -              1                                      8              ⁢                              n                3                                              ⁢                                    (              NA              )                        4                    ⁢          Δ          ⁢                                          ⁢          d                                    (        2        )            
As is obvious from this numerical formula (2), the third-order spherical aberration W lengthens in proportion to the variation Δd in the light transmission layer's thickness from the optimum material thickness. In other words, the greater the thickness of a light transmission layer becomes by multi-layering an information recording layer, the greater spherical aberration can be corrected. Therefore, in a conventional optical head, the movement distance of a collimating lens required to correct a produced spherical aberration also lengthens. As a result, in order to secure such a distance by which the collimating lens moves, a collimating-lens actuator needs to be larger. This presents a disadvantage in that the optical head becomes larger.